This invention relates to a surface acoustic wave apparatus and a method of production and adjustment of the same for providing superior skirt and out-of-band characteristics when used as a filter.
Conventional means for improving the skirt characteristic of the surface acoustic wave filter (the sharpness of the attenution characteristic at the edges of the frequency band of the filter) is a multistrip coupler which is described in, for example, IEEE Trans. on Microwave Theory and Techniques, Vol. MTT-21, No. 4, Apr. 1973, pages 206-215.
In the above prior art, in order to improve the skirt characteristic, it is necessary to increase the number of the interdigital transducer strips or fingers, and thus the chip size becomes large and the cost is inevitably increased. The relation between the skirt characteristic and the number of transducer strips is described in, for example, IEEE Trans. on Microwave Theory and Techniques, Vol. MTT-21, No. 4, Apr. 1973, pages 162-175.
Moreover, since the number of transducer strips must be increased in order to improve the skirt characteristic as described above, the radiation conductance of the interdigital transducer is increased, thus increasing the reflection which is determined by the conditions of the electrical load, or electrical reradiated wave (RW). Therefore, normally to suppress the reflection, the aperture of the interdigital transducer strips facing each other must be made narrow. In that case, the diffraction effect of the surface acoustic wave becomes more influential, causing the sidelobes of the filter characteristics to deteriorate.